Implementing and maintaining billing is my daily job for the last 6 years. I would also mention topics like: - Reporting (various data aggregation and audit re

Implementing and maintaining billing is my daily job for the last 6 years. I would also mention topics like: - Reporting (various data aggregation and audit reports) - High reliability, nothing hurts company more than unability to bill their customers - Rounding, this can be seen as a subset of "taxes" but it's much more complex - Locating user, also can be seen as a subset of "taxes" but user can have country A set in profile configurations, country B on credit card and country C geolocated from IP address - Timezones, issue everywhere but when we talk accounting it's super important - Talking to moronic payment gateway providers. This is the biggest ones. I would love to just flip Apple and Google like Linus flipped of nVidia. Proprietary, poorly documented, "find out yourself by weeks of experimenting" bullshit with no easy way of getting technical support even when you bring those companies millions per year. Things don't work, deprecate monthly or weekly and expect you to be always ready to make changes. Some implementation make zero sense at all with complete paradigm shift of handling payments like between Google in-app pay and Google subscriptions. But as my SO says "Don't cry, it pays well"